On control of the grid‐connected inverter (GCI) with LCL filter, the inverter‐side current model predictive control is adopted conventionally. The ultimate grid‐side current is controlled indirectly by control of inverter‐side current. The ideal scenario is that grid‐side current is directly controlled. However, the conventional control model is complicated and the calculation is heavy. This study proposed a novel direct grid‐side current model predictive control (GSC‐MPC) for GCI with LCL filter. Based on timing coordination of both forward and backward difference methods, a direct connection is established between the grid‐side output current and the inverter‐side output voltage. Meanwhile, a proper mathematical model is built for an improved model predictive control. The exact required voltage vector in the next sampling interval is predicted and calculated by this model. The output optimal voltage vector is modulated by space vector pulse width modulation technique to control the inverter. Furthermore, the proposed GSC‐MPC is rather robust to parameter variation. Simulation and experimental results present that the proposed GSC‐MPC can improve the current control performance effectively for GCI with LCL filter.

The modular multilevel converter (MMC) is one of the best potential candidates for high‐voltage direct current (HVDC). Various kinds of faults may occur in an MMC‐HVDC system. The system should be restarted by deblocking the converter after a temporary fault is cleared or a fault is isolated, thus recovering the power transmission as soon as possible. This study analyses the recovery process after the converter is deblocked. The analysis shows that the capacitor voltages of the three upper arms or the three lower arms can be rebalanced quickly, and the power transmission and the DC voltage can be recovered with the original main controller. However, the speed of rebalancing the capacitor voltages of the upper arm and the lower arm depends on the arm resistance, the modulation index and so on, and is probably slow. To improve the damping characteristic of the MMC, a dedicated supplementary controller is proposed based on the concept of ‘virtual arm resistance’. Time‐domain simulation studies based on PSCAD/EMTDC are performed to verify the analysis and the proposed controller.

This paper analyzes the operation characteristics of the modular multilevel converter when the arms are asymmetrical with different sub-module number caused by sub-module faults. The research reveals the problems caused by asymmetrical arms, including the asymmetrical capacitor voltages, the asymmetrical direct current (dc) components of the arm currents, the asymmetrical circulating currents, the dc current fluctuation, and the voltage offset at the dc or alternating current (ac) side. Then, it is demonstrated by the theoretical analysis that the series of problems can be solved by rebalancing the fundamental frequency components of the upper arm voltage and the lower arm voltage. Therefore, a fault-tolerant control strategy is proposed by introducing a fundamental frequency resonant controller to the traditional second-order frequency proportional resonant circulating current controller. Besides, the research shows that the coordination of the proposed control strategy and the negative sequence current suppressing control can keep the output voltages of the three phases balanced. A 201-level modular multilevel converter–high-voltage direct current time-domain simulation model is established based on PSCAD/EMTDC, and the simulation studies confirm the efficiency of the proposed controller under balanced and unbalanced ac grid conditions. Copyright © 2015 John Wiley & Sons, Ltd.

In the traditional model predictive control(T-MPC), only inverter side current of LCL inverter is treated as a control target. Grid side current is controlled indirectly by controlling the inverter side current directly. The control performance of the algorithm can be further improved. In this paper, a complete model predictive control algorithm(C-MPC) which take the grid side current as control target directly is proposed. The equivalent mathematical model and the equivalent circuit model are established based on active damping of capacitive current proportional feedback, then system predictive control model is established by the equivalent mathematical model and the circuit model. In order to make the output current track the directive current accurately at the next moment, the output voltage vector needed at this moment is calculated by the predictive control model. Finally, the required voltage vector is reproduced through space vector modulation(SVM). The simulation results and experimental measurements verify the developed control schemes and show high quality of grid side current (low THDi value).

Modular multilevel converter (MMC) is widely employed in high-voltage direct current (HVDC) systems for the long-distance renewable energy transmission, where the larger submodule (SM) capacitors significantly increase its size, weight and cost. Conventional capacitor voltage fluctuation suppression methods, such as composite harmonic injection (CHI) strategies, can achieve lightweight MMC. However, these approaches often neglect the dynamic constraints between harmonic injection parameters and their coupled effect on modulation wave, which not only leads to suboptimal global solutions but also increases the risk of system overshoot. Therefore, this paper proposes a comprehensive CHI parameters optimization method to minimize capacitor voltage fluctuations, thereby allowing for a smaller SM capacitor. First, the analytical expression of SM average capacitor voltage is developed, incorporating the injected second-order harmonic circulating current and third-order harmonic voltage. On this basis, an objective function is defined to minimize the sum of the fundamental and second-order harmonic components of the average capacitor voltage, with the harmonic injection parameters and modulation index as optimization variables. Then, these parameters are optimized using a particle swarm optimization (PSO) algorithm, where their constraints are set to prevent modulation wave overshoot and additional power loss. Finally, the optimization method is validated through a ±500 kV, 1500 MW MMC-HVDC system under various power conditions in PSCAD/EMTDC (version 4.6.3). In addition, simulation results demonstrate that the proposed method can achieve a 13.33% greater reduction in SM capacitance value compared to conventional strategies.

The oscillation mechanism of the hybrid cascaded DC system is very complex. This is mainly due to the fact that the conventional DC converter and the flexible DC converter are directly connected on the DC side, resulting in a resonant circuit on the DC side. Herein, this work carry out research on the oscillation mechanism and suppression strategy for the hybrid cascade system. In this work, the DC side frequency domain impedance model of the hybrid cascade system is established and the oscillation mechanism of the DC side of the hybrid cascade DC system is analyzed. The method of suppressing the hybrid cascade dc-side oscillations by configuring the detector, optimizing the switching frequency control strategy and optimizing the modulation control strategy is explored. This paves the way for suppressing DC side oscillations.